Sequential Reduction Pathway

Anaerobic Reduction

In contrast to TNT, sequential reduction of RDX occurs mostly under anaerobic conditions, as initially presented by McCormick et al. (1981). This pathway proceeds through reduction of the nitro groups to nitroso derivatives by subsequent two-electron transfer steps, followed by accumulation of hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX), hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine (DNX), and hexahydro-1,3,5-trinitroso-1,3,5-triazine (TNX) derivatives. Again in contrast to TNT, the nitroso derivatives do not tend to undergo further reduction to stable detectable product. Although this degradation pathway leads to the desired decrease in RDX concentrations in the contaminated environment, the accumulation of the nitroso derivatives themselves is undesirable due to their toxicity (Zhang et al. 2006). Nevertheless, the nitroso derivatives can further degrade via ring cleavage, after being transferred to the corresponding hydroxylamine metabolites, and produce more simple compounds that can undergo complete mineralization (McCormick et al. 1981).

Table 5.4 Microbial isolates of RDX degraders

Isolate

Conditions

Suggested pathway, indicative intermediates

Reference

Bacillus strains HPB2, HPB3

Denitrifying

Pathway not defined, products not identified

Singh et al. (2009)

Acetobacterium malicum strain

Anaerobic

Denitration of RDX and MNX followed by ring cleavage.

Adrian and Arnett (2004)

HAAP-1

Detection of MEDINA

A. paludosurn

Anaerobic

Ring cleavage of RDX. nitroso derivatives were not detected. Other intermediates were not identified

Sherburne et al. (2005)

Citrobacter freundii

Anaerobic

Sequential reduction. Detection of MNX, DNX, TNX

Kitts et al. (1994)

C. freundii NS2

Anaerobic

Sequential reduction. Detection of MNX, DNX, TNX

Kitts et al. (1994)

Clostridium acetobutylicum

Anaerobic

Reduction and formation of amino products. Detection of mononitroso-, monohydroxylamino-, mononitrosomonohydroxylamino-, monoamino-, diamino-, and triamino-compounds

Zhang and Hughes (2003)

C. bifennentans HAW-1

Anaerobic

(1) Reduction to MNX; (2) MNX denitration followed by ring cleavage

Zhao et al. (2003a, b)

Clostridium sp. EDB2

Anaerobic

Denitration pathway. Products were not reported

Bhushan et al. (2004)

Clostridium sp. HAW-1,

Anaerobic

(1) Reduction to MNX; (2) MNX denitration followed by ring

Zhao et al. (2003b)

HAW-G3, HAW-G4,

cleavage

HAW-E3, HAW-HC 1,

and HAW-ES2

Clostridium sp. HAW-E3

Anaerobic

(1) Reduction to MNX; (2) MNX denitration followed by ring cleavage

Zhao et al. (2003b)

Clostridium sp. HAW-EB17

Anaerobic

Sequential reduction, denitration and ring cleavage of RDX and MNX. Detection of MNX, DNX, TNX. With resting cells, detection of NDAB and MEDINA

Zhao et al. (2004a)

Clostridium sp. HAW-G4

Anaerobic

(1) Reduction to MNX; (2) MNX denitration followed by ring cleavage

Zhao et al. (2003b)

Clostridium sp. HAW-HC1

Anaerobic

(1) Reduction to MNX; (2) MNX denitration followed by ring cleavage

Zhao et al. (2003b)

Isolate

Conditions

Suggested pathway, indicative intermediates

Reference

Desulfovibrio sp. HAW-EB18

Anaerobic

Sequential reduction, denitration and ring cleavage of RDX and MNX. Detection of MNX, DNJC TNX. With resting cells, detection of NDAB and MEDINA

Zhao et al. (2004a)

Desulfovibrio sp.

Anaerobic

Pathway not defined. Sequential reduction or direct ring cleavage were excluded as major pathways following low nitroso derivative concentrations and no detection of MEDINA

Arnett and Adrian (2009)

Enterobacter cloacae

Anaerobic

Sequential reduction. Detection of MNX, DNX, TNX

Kitts et al. (2000), Pudge et al. (2003)

Fusobacteria isolate HAW-

Anaerobic

Sequential reduction, denitration and ring cleavage of RDX

Zhao et al. (2004a, b)

EB21

and MNX. Detection of MNX, DNX, TNX. With resting cells, detection of NDAB and MEDINA

Geobacter metallireducens

Anaerobic

Reduction to MNX, RDX and MNX denitration and ring

Kwon and Finneran (2008)

strain GS-15

cleavage. Detection of MEDINA

Gordonia sp. KTR9

Aerobic

Pathway not defined, products not identified

Thompson et al. (2005)

Gorodnia strain YY1

Aerobic

RDX denitration and ring cleavage. Detection of NDAB

Ronen et al. (2008)

Klebsiella pneumoniae SCZ1

Anaerobic

(1) Reduction to MNX; (2) MNX denitration followed by ring cleavage

Zhao et al. (2002)

Methylobacterium extorquens

Aerobic

Reduction to MNX followed by denitration and ring-cleavage. Detection of MNX, MEDINA

Van Aken et al. (2004)

M. organophilum

Aerobic

Reduction to MNX followed by denitration and ring-cleavage. Detection of MNX, MEDINA

Van Aken et al. (2004)

M. rhodesianum

Aerobic

Reduction to MNX followed by denitration and ring-cleavage. Detection of MNX, MEDINA

Van Aken et al. (2004)

Methylobacterium sp. BJ001

Aerobic

Reduction to MNX followed by denitration and ring-cleavage. Detection of MNX, MEDINA

Van Aken et al. (2004)

Morganella morganii B2

Anaerobic

Sequential reduction. Detection of MNX, DNX, TNX

Kitts et al. (1994)

M. morganii

Anaerobic

Sequential reduction. Detection of MNX, DNX, TNX

Kitts et al. (1994)

(continued) g

(continued) g

Isolate Conditions Suggested pathway, indicative intermediates Reference

Providentia rettgeri B1

Anaerobic

Sequential reduction. Detection of MNX, DNX, TNX

Kitts et al. (1994)

P. rettgeri

Anaerobic

Sequential reduction. Detection of MNX, DNX, TNX

Kitts et al. (1994)

Pseudomonas putida HK-6

Aerobic

Pathway not defined, products not identified

Cho et al. (2008)

Pseudomonas strain HPB1

Denitrifying

Pathway not defined, products not identified

Singh et al. (2009)

Rliodococcus rliodochrous sp.

Aerobic

Two denitration steps followed by ring cleavage. Detection of

Seth-Smith et al. (2002)

11Y

NDAB

Rliodococcus sp. DN22

Aerobic

Two denitration steps followed by ring cleavage. Detection of

Coleman et al. (1998), Fournier

NDAB

et al. (2002), Bhushan et al.

(2003b)

Rliodococcus sp. YH1

Aerobic

Two denitration steps followed by ring cleavage. Detection of

Nejidat et al. (2008)

NDAB

Serratia marcescens

Anaerobic

Sequential reduction. Detection of MNX, DNX, TNX

Young et al. (1997)

Shewanella halifaxensis HAW-EB4

Anaerobic

Sequential reduction. Detection of MNX, DNX, TNX

Zhao et al. (2004a, 2006)

S. sediminis HAW-EB3

Anaerobic

Sequential reduction. Detection of MNX, DNX, TNX

Zhao et al. (2004a, 2005)

Shewanella sp. HAW-EB1,

Anaerobic

Sequential reduction, denitration and ring cleavage of RDX

Zhao et al. (2004a)

HAW-EB2, HAW-EB5

and MNX. Detection of MNX, DNX, TNX, NDAB,

MEDINA

Stenotrophomonas maltophilia

Aerobic

Pathway not determined, detection of methylene-N-

Binks et al. (1995)

PB1

(hydroxymethy 1 )-hy droxy lamine-N' -

(hydroxymethyl )nitroamine

Williamsia sp. KTR4

Aerobic

Pathway not defined, products not identified

Thompson et al. (2005)

Aerobic Reduction

Under aerobic conditions, the formation of nitroso derivatives is normally not observed. This may be expected from thermodynamic considerations, where the calculated E0 shows a decrease from trinitroaromatics to nitramine, suggesting thermodynamic control of the reduction of RDX under aerobic conditions (Uchimiya et al. 2010).

Only a few exceptions indicate formation of mononitroso derivatives under aerobic conditions: the first, reported for incubation of the white-rot fungus P. chrysosporium with RDX (Sheremata and Hawari 2000) which showed formation of MNX. This was followed by ring cleavage and the subsequent formation of methylene dinitramine (MEDINA). The second, presented by Van Aken et al. (2004), reported the detection of MNX during RDX biodegradation under aerobic conditions by a phytosymbiotic Methylobacterium sp. that was associated with poplar tissue. Latter on these authors observed a polar metabolite which was produced by the ring cleavage of MNX, and had the molecular formula of MEDINA. The detection of di- or tri-nitro derivatives under aerobic conditions has never been documented.

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